1. Field of the Invention
The present invention relates to an optical module for optical communication, and in particular, relates to an optical module with a CAN package.
2. Description of the Related Art
An Optical transceiver for optical communication, such as 10 gigabit small form factor pluggable (XFP) optical transceiver, often contains an optical module with a CAN package. The optical module is a coaxial type and enables high speed transmitting and/or receiving for optical communication. The optical transceiver converts an optical signal into an electric signal and/or an electric signal into an optical signal. The CAN package is one of basic packages that have airtight property, and is often used as a packaging for a laser diode installed in a compact disc drive (CDD).
FIG. 1A shows a side view of an optical transceiver 100 as an example of optical transceivers. The optical transceiver 100 has an optical module 101, a mount board 105 and a flexible print substrate 104. The optical module 101 is a coaxial type and enables high speed transmitting and/or receiving for optical communication. The optical module 101 contains a stem portion 121, a cap 122 and a connector 123 to which one end of an optical fiber cable is connected. The stem portion 121 contains a stem 103 and leads 102. One side of the cap 122 is fixed to the front end of the stem 103 by the resistance welding such that the cap 122 covers the front surface 131 of the stem 103. The connector 123 is attached to the opposite side of the cap 122. The cap 122 and the stem 103 form a CAN package of the optical module 101. The CAN package is an airtight packaging.
FIGS. 1B and 1C show the configuration of the stem portion 121 in the case that the optical module 101 is configured as a receiver. FIG. 1B shows a front view of the stem portion 121. FIG. 1C shows a cross-sectional view of the stem portion 121. As shown in FIGS. 1B and 1C, the stem portion 121 contains chip parts 106, a light receiving element 107, a pre-amplifier 108 and bonding wires 109. The chip parts 106, the light receiving element 107 and the pre-amplifier 108 are mounted on the front surface 131. The stem 103 has holes penetrating the stem 103 from the front surface 131 to the back surface 132 of the stem 103 and inner cylindrical surfaces, each of which surrounds each of the holes. Each of the leads 102 extends through each of the holes such that a gap 110 exists between that lead 102 and the inner cylindrical surface surrounding that hole. The gap 110 is entirely filled with sealing glass that seals the gap 110. Each of the leads 102 has first end which protrudes beyond the front surface 131 into the inside of the CAN package and second end which protrudes beyond the back surface 132 to the outside of the CAN package. The bonding wires 109 connect among the chip parts 106, the light receiving element 107, the pre-amplifier 108 and the first ends of the leads 102. The second ends of the leads 102 are connected to a circuit on the mount board 105 through lines on the flexible print substrate 104.
The box in FIG. 1C indicates the region 124 in the stem portion 121. The region 124 contains the gap 110 filled with sealing glass, a center portion of the lead 102 surrounded by the gap 110 and a gap-surrounding portion of the stem 103 surrounding the gap 110. The center portion of the lead 102 is placed between the first and second ends of the lead 102. The region 124 can be regarded as a coaxial line 111, as shown in FIG. 1D. The coaxial line 111 is an equivalent circuit model of the region 124. The gap 110 entirely filled with sealing glass is correspond to a dielectric between inner and outer conductors of the coaxial line 111. The center portion of the lead 102 is correspond to the inner conductor of the coaxial line 111. The gap-surrounding portion of the stem 103 is correspond to the outer conductor of the coaxial line 111. In this case, a dielectric constant ∈r(111) of the dielectric in the coaxial line 111 is equal to that of glass. Thus, a characteristic impedance Z0(111) of the coaxial line 111 is represented by the following equation:
                                          Z            0                    ⁡                      (            111            )                          =                              138            ×                          Log              ⁡                              (                                  a                  /                  b                                )                                                                                        ɛ                r                            ⁡                              (                111                )                                                                        (        1        )            wherea is the inner diameter of the coaxial line 111 (the diameter of the lead 102).b is the outer diameter of the coaxial line 111 (the outer diameter of sealing glass which fills the gap 110).∈r(111) is the dielectric constant of the dielectric in the coaxial line 111.The equation (1) is appropriate when the frequency of the signal transmitted in the coaxial line 111 is about 10 GHz.
The term “dielectric constant” indicates relative dielectric constant.
An operation of the optical transceiver 100 is described below. The light receiving element 107 receives an optical signal through the cap 122, converts the optical signal into an electric signal and outputs the electric signal to the pre-amplifier 108. The pre-amplifier 108 amplifies the electric signal and outputs the amplified electric signal, which is a high frequency signal, to the circuit on the mount board 105 through a plurality of the region 124 and the lines on the flexible print substrate 104.
The optical transceiver 100 is required to have a high frequency characteristic that enables a high bit rate transmission at bit rate of 10 GBps or more. Here, the optical transceiver 100 can be regarded to have two circuits. The first circuit contains the chip parts 106, the light receiving element 107, the pre-amplifier 108, the bonding wires 109 and the plurality of the region 124. The second circuit contains the lines on the flexible print substrate 104 and the circuit on the mount board 105. In order to attain the high frequency characteristic, it is important to attain impedance matching between the two circuits and to suppress parasitic inductance. The impedance matching can be attained by adjusting the impedance of the region 124. A parasitic inductance, which is caused by one of the bonding wires 109 that connects the pre-amplifier 108 and one of the first ends of the leads 102, can be suppressed by setting that bonding wire 109 short. In order to connect the pre-amplifier 108 and the first end of the lead 102, the longer length of the bonding wire 109 is required when the outer diameter b is the larger.
The optical module 101 is required to have a small size in order to miniaturize the optical transceiver 100. The small size of the optical module 101 can be obtained by setting the diameter c of the stem 103 small. The small diameter c can be obtained by setting the outer diameter b small. The smaller outer diameter b is required to reserve the wider area for mounting the chip parts 106 on the front surface 131.
Some conventional techniques, which are related to the present invention, are described below.
Japanese Laid Open Patent Application (JP-P2001-298217A) discloses an optical module. The optical module is designed such that a flexible print substrate, which contains a light receiving element, a light emitting element, electronic parts relevant to light reception and electronic parts relevant to light emission, and two reinforcement plates are placed in a body, and 10 lead pins are extended from the flexible print substrate to outside the body. The electronic parts relevant to light reception are placed on the first portion of the flexible print substrate and the electronic parts relevant to light emission are placed on the second portion of the flexible print substrate. The flexible print substrate and the reinforcement plates are placed in the body so that the flexible print substrate is bent to make the first and the second portions layered, and the reinforcement plates are inserted into the gap between the layered portions.
Japanese Laid Open Patent Application (JP-P2003-332667A) discloses a semiconductor laser module. Impedance of a glass sealing portion for sealing a lead pin that penetrates through a penetration hole formed in a stem base of the semiconductor laser diode is adjusted to a predetermined impedance by adjusting its dimension. Since a resistance element is connected in series to a laser diode mounted on the stem base, the matching with the impedance of the glass sealing portion is attained. Also, a connecting member having a transmission line is placed between the lead pin and the laser diode. The matching with the impedance of the glass sealing portion is attained by adjusting the shape and property of the transmission line.
Japanese Laid Open Patent Application (JP-P2004-311923A) discloses an optical semiconductor element package. The optical semiconductor element package contains a stem and a signal supply lead terminal. The stem has a front surface and a back surface. A penetration hole is formed in the stem to penetrate the stem from the front surface to the back surface. The signal supply lead terminal penetrates through the penetration hole such that the signal supply lead terminal is insulated from the penetration hole by an insulator between the penetration hole and the signal supply lead. The signal supply lead has a first portion which is placed in the penetration hole and a second potion which protrudes from the front surface. Then, so as to reduce the difference between characteristic impedance of a transmission line constituted by the penetration hole, the insulator and the first portion; and characteristic impedance of a transmission line constituted by the second portion, a grounded conductor is installed closely to the second portion.
Japanese Laid Open Patent Application (JP-P2005-12224A) discloses an optical receiving module with a TO-Can structure. The optical receiving module with the TO-can structure is characterized by including: a stem where holes penetrating the stem between both surface of the stem are formed; and a photo diode which is located on the front surface of the stem and converts an optical signal inputted therein into a current and further including a trans-impedance amplifier, signal leads, ground leads and waveguides. The trans-impedance amplifier is located on the front surface of the stem and converts the current, which is outputted from the photo diode, into high frequency signals having phases opposite to each other, and amplifies the signals and then outputs the amplified signals through respective output terminals to outside. Each of the signal leads penetrates through each of the holes. The signal leads output the amplified signals to the outside. The stem is grounded to the outside of the optical receiving module through the ground leads extending from the back of the stem. The waveguides are fixed to the predetermined position on the front surface of the stem in order to attain the impedance matching between the trans-impedance amplifier and the leads, and transmit the amplified signals outputted from the respective output terminals of the trans-impedance amplifier through the corresponding electric routes to the respective leads, respectively.